1. Field of the Invention
The present invention relates to a lithium nickel cobalt manganese composite oxide cathode material, more specifically to a lithium nickel cobalt manganese composite oxide is basically formed secondary particles consisting of aggregates of fine primary particles, each having a structure with different chemical composition of primary particles from the surface toward core of each of the secondary particles.
2. The Prior Arts
Recently, lithium battery are broadly applied in notebook computers, cell phones, PDAs, video cameras, digital cameras, mini CD-ROMs, hand-held terminals, and Bluetooth™ earphone, etc. The 3C products in the coming futures demand not only the characteristics of thin, light, and compact but also require high energy storage, high resolution, and dynamic true color in product specification. Therefore, high energy density, thinner and portable lithium battery is the current trend of development. In addition, in the trend of environmentally technology and green energy, electric bicycles, electric motorcycles and electric hybrid cars will be developed in mass production in the near future. These transport vehicles require the power sources of batteries, which have characteristics of high power, high capacity, high safety but low cost.
Cathode material used on a lithium battery not only affects the performance of the battery, but also influences the safety of the lithium battery. Therefore, a good cathode material for applying in the lithium battery is defined to have high specific capacity and high thermal stability, which means high safety. Lithium cobalt oxide (LiCoO2) has popularly used in lithium battery for many years; however, the raw material is expensive and toxicant to the environment. In addition, the capacity and performance of LiCoO2 is difficult to be improved. Lithium nickel oxide (LiNiO2) with a high capacity, low price and less toxicity is developed for replacing LiCoO2. However, the LiNiO2 is unsafe and poor cycleability. Therefore, LiNiO2 is difficult to use in lithium battery. Lithium manganese oxide (LiMn2O4) provides better safety characteristic, but the specific capacity is too low to satisfy the battery demand of high capacity. Partial substitution of Ni by other metal cations has been made to enhance the LiNiO2 electrochemical performance. In particular, the solid solution, Lithium nickel cobalt manganese composite oxides (LiaNi1-b-cCobMncO2), has been suggested as an alternative to LiCoO2 as it combines some of the benefits of LiNiO2 (capacity) with those of LiCoO2 (stability) and LiMn2O4 (safety).
Recently, the lithium nickel cobalt manganese composite oxide cathode material has widely utilized in many commercialized products. But a key problem of lithium nickel cobalt manganese composite oxide is that it is difficult to obtain a high-safety and high-capacity material which formula is with the higher manganese and the higher nickel content at the some time. In order to resolve the problem, some researcher select lithium nickel cobalt manganese composite oxide with low manganese content to decrease the capacity loss, and dopes other metal element into the lithium nickel cobalt manganese composite oxide to enhance the stability of structure. Although the stability of structure has much improved and provides better safety than pristine lithium nickel cobalt manganese composite oxide, however, the capacity of the cathode is still reduced.
In these years, some researchers provide a method to coat a nano-protective layer on the surface of lithium nickel cobalt manganese composite oxide in order to prevent the HF attack from the electrolyte, thereby ensuring the structure of material. However, the method can decrease the exothermic heat, but can not raise the thermal-decomposition temperature. In addition, it is hard to control the thickness of coated layer and mass production.
Other researchers provide a core-shell complex structure of cathode material, which uses the lithium nickel cobalt manganese composite oxide as a core of cathode material, and a thermal-stability cathode material covered on the surface of the lithium nickel cobalt manganese composite oxide to form a protective shell. For example, the thermal stability cathode material is lithium iron phosphate and the thickness of the protective shell is 1˜2 μm. This structure greatly improves the safety of material; however, the interface resistance inside the material is also increase, such that the discharge performance is decreased under high-rate test. Moreover, the synthesis of material with core-shell structure is hard to control the quality in mass production.